Ultra-high frequency selective mode directional coupler



S. E. MILLER May 29, 1956 ULTRA-HIGH FREQUENCY SELECTIVE MODE DIRECTIONAL. COUPLER 4 Sheets-Sheet l Filed Sept. 5, 1951 LOAD ClRCU/T /N l/E/v rop 5. E M/L L E R May 29, 1956 ULTRA-HIGH FREQUENCY SELECTIVE MODE DIRECTIONAL COUPLER Filed Sept. 5, 1951 A MPL/ TUDE F s. E. MILLER 2,748,350

4 Sheets-Sheet 2 /Nl/ENTOR S. E. 'M/LLER ATTORNEY May 29, 1956 s. E. MILLER 2,748350 ULTRA-HIGH FREQUENCY SELECTIVE MODE DIRECTIONAL COUPLER Filed Sept. 5, 1951 4 Sheets-Sheet 3 56\ F/G. 7 e

, L s/GNAL 75,0 77- MODULA TOR 60 DE MODULA TOR 5/ TEN /0 5 7 MODUL A TOR OR y DE MODUL/1 TOR `/56 s/GNAL 75,0 F/G. 8

MODULA TOR 0R DEMODULA TOR 5/ MoouLAroR of? 57 DEMODULA TOR s/GNAL /Nl/EA/ TOR 55 2 5. E. MILLER CROSSED OU TPU T TE/l 8 TMO, OUTPUT A TTORNE V S. E. MILLER May 29, 195,6

ULTRA-HIGH FREQUENCY SELECTIVE MODE DIRECTIONAL COUPLER 4 Sheets-Sheet 4 Filed Sept. 5 1951 /NI/E/VTOR 5. E. M/LLER BV www' A7' ORA/EV' ULTRA-HIGH FREQUENCY SELECTIVE MODE DIRECTIONAL COUPLER ,Stewart E. Miller, Middletown, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application September 5, 1951, Serial No. 245,210

34 Claims. (Cl. S33-10) This invention relates to wave transmission systems for the transmission of electrical wave energy along multimode transmission lines and, more particularly, to devices having special transmission characteristics dependent upon the particular mode of transmission of said wave energy.

Such guided waves, as is well known in the microwave transmission art, are capable of transmission in an,v infinitely large number of forms or modes, each mode being distinguished by the characteristic configuration of the component electric and magnetic fields comprising the waves.

These waves have been divided into two broad classes. In one class the electric component of the wave is transverse to the metallic pipe guide, and at no point does it have a longitudinal component. The magnetic component, on the other hand, has both transverse andlongtudinal components. This class has been designated at ,transverse electric waves or TE Waves. In the other class, the magnetic component is transverse to the pipe and at no point does it have a longitudinal component, but the electric component has in general both transverse and longitudinal components. This class has been designated as transverse magnetic waves or TM waves.

The waves in each of these classes have beenV further identified and distinguished from each other by their mode or the pattern of wave energy distribution as it appears in the cross-section of the wave guide. A complete discited States Patent cussion of wave mode may be found in any standard textbook of microwaves and microwave guides. For the purpose of the present disclosure, the usual convention is herein adopted of designating a transverse electric wave TEmn where, in a rectangularwave guide, m represents the number of half period variations of the transverse component encountered in passing across the widthof the wave-guide cross-section, and n represents the numberof Vhalf periods of transverse components encountered in passingacross the height of the wave guide; vand in a circular or cylindrical wave guide, m representsthe number of whole periods of the transverse component encountered in running aroundV the circumference of the crosssection, and n represents the number of half periods encountered in passing along the radius of the wave-guide cross-section.

For example, in a rectangular wave guide, TEin represents a wave having a one-half electric period variation across the width of the guide, and, since the eld is uniform, there is no variation across the height of the guide.

This is commonly known as the dominant mode wave.

in certain characteristics that render one mode particu- 2,748,3st Patented May 29, 1956 larly suitable for one use in electrical communication systems and another mode suitable for another use. For example, the circular electric wave, when propagated through a circular metal pipe of a given diameter, suffers progressively less attenuation as the frequency is increased. Because of this low attenuation, the TEoi wave is particularly adapted for long distance transmission systems. On the other hand, the TEio wave or dominant mode wave is easily generated by presently known generators, does not easily degenerate into other modes, and in addition, is most favorable for amplification, modulation or demodulation. Similar advantages and disadvantages might be cited for other modes.

It is, therefore, an object of the present invention to transfer wave power of a given mode in a first transmission line, suitable for one purpose therein, from said rst line and to launch it as wave power of another mode suitable for another purpose in a second transmission line.

Another object of the invention is to transfer a predetermined portion or the entire portion of energy in the first mode into energy in the second mode.

As is well known, the size of a shielded transmission line may be so chosen that the line will support only one mode, usually the dominant mode of propagation. However, as the size of the line is increased, it becomes multimode and may support simultaneously wave energy in a plurality of modes of propagation.

It is an object of the present invention to launch wave energy of only specifically selected mode configuration in a multimode transmission line.

It is a further object of the present invention to select or filter wave energy having specifically selected mode configurations in a multimode transmission line from energy having other mode configurations.

It has been determined, in accordance with the invention, that when two transmission lines of uniform characte'ristics are coupled, in the particular manner to be described, ,over a longitudinal length thereof, that wave energy will be transferred between the lines when the velocity of propagation of the wave energy in each of the two lines is equal. It has further been determined that when the velocities of propagation of wave energy in the two lines are unequal, no energy Will be transferred between. the lines when the length of the coupling interval bears the particular relation to behereinafter specilied, to the difference between the velocities in the two lines and to the Fourier transform of the particular coupling distribution, inasmuch as the velocity of propagation ofvwave energy in any particular mode is uniquely determined by the physical dimensionslof the transmission line, these physical dimensions and the length of the coupling distribution are chosen in accordance with the invention to couple, reject, ilter or select any desired mode or modes of wave energy propagation.

Certain classes of modes have individual mode configurations for which the velocity of propagation is the same as a corresponding individual mode configuration in another class of modes. Special features of the invention reside in the means provided to separate modes by classes and therefore to separate the individual mode configurations having like velocities of propagation.

.Further features of'the invention reside in combination of two or more speciiically chosen mode transducers to launch two or more intelligence-bearing signal waves in different modes of propagation for transmission simultaneously in a single wave-guide system.

Other objects, aspects and features of the invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings in which:

Fig. l shows in pictorial representation a high frequency selective mode transducer, in accordance with the invention;

Fig. lA is a cross-sectional view of the transducer or Fig. l taken as indicated;

Fig. 2 is a diagrammatic representation of the transducer of Fig. l, given for purposes of illustration;

Fig. 3 is a representation of certain critical characteristics of the transducer of Fig. 1, ,given for purposes of illustration;

Fig. 4 shows a modified embodiment of the transducer of Fig. l;

Fig. 5 is a representation of certain critical characteristics of the transducer of Fig. 4, given for purposes of illustration;

Fig. 6 illustrates a modification of the selective mode transducer of Fig. 1, whereby dominant mode TEio wave energy in a rectangular guide is transformed into TMm wave energy in a circular guide;

Fig. 7 shows a multichannel transducer, whereby two or more signal channels may be transmitted Vor received in a single round wave guide by cross-polarized TEii wave energy;

Fig. 7A is a cross-sectional View of Fig. 7, taken as indicated;

Fig. 8 shows a multichannel transducer, whereby two or more signal channels may be transmitted or Vreceived in a single'round wave guide by energy in the TEn and TMm modes;

Fig. 9 shows a multichannel transducer, whereby each of a plurality of signal channels maybe transmitted or received in one of the lower order circular electric kwave modes; Y

Fig. 9A is a left-hand end view of Fig. 9;

Fig. 10 shows a multichannel transducer in accordance with the invention whereby a `plurality of dominant mode TEinv signal channels of different frequencies may be transmitted or received as TEoi circular electric signals at corresponding frequencies;

Fig. 10A is a left-hand end view of Fig. l0; and

Fig. 11 shows a further alternative embodiment of the invention employing the rectangular wave-guide modes.

ln more detail, Fig. l shows a high frequency `Selective mode transducer, in accordance with the invention, for transforming dominant mode TEio wave energy in a rectangular wave guide into TEoi circular electric wave energy in a circular `wave guide. The transducer of Fig. 1 comprises a rst section 13 of shielded transmission line for guiding wave energy 'which is, as shown on the cross-sectional view of Fig. 1A, a dominant mode rectangular wave guide having an "internal wide dimension a and a narrow dimension b. Wave guide 13 is provided with terminal flange connections 26 and 27 at each of its ends bent at right angles to the guide `for .convenience in mechanical assembly. 'Loca-ted adjacent to wave guide 13 and having a portion of its length contiguous to a portion of guide 13 Vis a second shielded transmission line 10 for guiding wave energy, which is a circular cross-sectional wave guide of radius r, having terminal connections 11 kand 12 at each vof its ends for mechanical assembly. The reverse end, toward terminal 11, of guide 10 is terminated in a reiiectionless manner by its characteristic impedance as indicated diagrammatically by termination 21 of electrical high loss material. The forward end, toward terminal 12, of guide 10 is connected to a load circuit adapted to utilize the generated TEO; waveenergy. This load 20 may represent, for example, a long distance circular transmission line. A source 25 of TEio dominant mode electromagnetic wave energy is shown connected to terminal 2.6 of guide 13. The other end of guide 13 is terminated in a reectionless manner 'by' its characteristic impedance as indicated diagrammatically by termination 22 of electrical high loss material.

Guides 10 and 13 are coupled in their contiguous poition by like equally sized and equally spaced apertures 16 extending through the wall 14 of rectangular guide 13 'by f'Equation 1. guide 10 have the same cut-off wavelength, and there I4 and the adjacent wall portion 15 of guide 10. The wall thickness of guide 10'is cut away along the portion thereof contiguous to guide 13 so that portion 15 is flattened to iit snugly against wall 14 of the rectangular guide 13. The coupling apertures 16 are distributed along a total array length L measured along the longitudinal length of the guides from the centers of each of the end apertures. The Vapertures 16 are illustrated as elliptically shaped; however, within the limits set by the considerations to be detailed hereinunder, the shape -of these apertures may be of any shape vconsistent with good coupling. The relative cross-sectional dimensions of guides 10 and 13 are chosen so that the guide wavelength of TEoi wave energy 'in guide -10 is equal 'to the guide 1wavelength of TEio wave energy in guide 13. The significance of this relation will be brought out fully in the discussion which follows. In this connection it should first be noted that the guide wavelength kg for a particular mode of wave energy propagation in any guide depends directly upon the cut-off wavelength Ac of the Iguide in accordance with the following relation:

l=* M2-M2 where M fis ,the free space wavelength. 'It is thus `seen that 'if the cut-oit wavelength Yfor a particular mode in each of two guides is made equal, the guide wavelength of wave energy in the two guides will be equal, regardless of the particular frequency of the energy. These considerations are fully discussed in any standard .textbook on wave-,guide transmission, such as Southworth, Principles and Applications of vWaveguide Transmission," 19,50.

`It should further be noted that the cut-ott wavelength )te in a rectangular guide for any TEmn or Tlvlmn wave mode depends upon the physical dimensions of the wave guide and may be expressed:

in which either m or n may be zero. Thus, -for the TEio wave in which m equals 1 and n equals zero, the cutofi wavelength `lm=2a and the ,guide wavelength Ag is accordingly proportioned by Equation 1.

In .circular wave guides, the cut-off wavelength Ac lfor a particular mode is determined by the radins .r of the guide and 'is expressed:

in which kmn is Vthe Bessel function lconstant for the particular mode of transverse magnetic or transverse electric waves. A complete discussion of the 'Bessel function and its derivation may be found in any standard textbook on wave-guide transmission, 'but it will suihce here vto give the values of kmn for the lower order modes.

For transverse magnet-ic waves:

Thus for the ,IEoi wave,

and the guide wavelength Ag is accordingly proportioned In order then -that 7the T-Eoi wave in with usual practice for rectangular wave-guide crosssections, the dimension b of guide 13 may be equal to Having thus proportioned the physical dimensions of guides and 13 so that the guide wavelength for the TEoi mode in guide 10 is equal to the guide wavelength for the TE1o mode in guide 13, at least the energy coupled through each of the coupling apertures which happens to appear in guide 10 in the TEoi mode will add in phase for transmission to load 20, as will be demonstrated hereinafter, regardless of the aperture spacing or the length of the coupling array. But since the necessary dimensions of guide 10 render this guide multimode and, in all practical cases, capable of supporting all of tive other modes in addition to the desired TEoi mode, it would be expected that a substantial portion of the original TEio energy would appear in unwanted spurious modes in guide 10. An inspection of the above table of Bessel function constants will show that these five possible spurious modes would be the TMoi, TM11,V TEai, TE21 and TE11.

Reserving special consideration of the TMu mode for full treatment hereinafter, all of the remaining spurious modes are substantially suppressed in the forward direction of transmission, in accordance with the invention, by making the length L of the coupling array at least equal to substantially a certain length determined by the coupling distribution and the phase velocity difference between the desired mode and the modes to be discriminated against. This relation is The quantity While not strictly a velocity quantity is proportional to the velocity difference between the waves M and k2 and for convenience herein will be referred to as the velocity difference function. In the particular embodiment of Fig. 1, M is the guide wavelength of the TEai wave calculated in accordance with Equations l and 3, and 0 is equal to radians, as will be shown, for a coupling distribution comprising eight equal point couplings. This particular relation is indicated by the expression' on Fig. 1,.y

The reasons underlying the choice of these particular dimensions for the selective mode transducer of Fig. l will most easily be understood upon a consideration of the diagrammatic representation of this structure in Fig. 2.- On Fig. 2 are shown two identicaltransmission lines 1 and 2 corresponding, respectively, to lines 10 and 13 of Fig. `1. These transmission lines are assumed parallel and the direction of propagation is along the x-axis. The region in which coupling exists, corresponding to the array 6 length L of apertures 16, is designated in FigQ 2 by the interval from The coupling distribution, or the variations of coupling between the lines in the interval is described by the function ga(x). Assume further that the exciting wave generated by source 25 is traveling to the right in line 2, and that the fraction of this energy transferred to line 1 is negligible. Thus, the voltage wave on line 2 is in which the phase reference is taken as that at The voltage E(x) in series with line 1 due to the coupling represented at any distance x intermediate 2r L -i- --l-z p B(w)=(w)E2= (w)e MQ (6) The eifect of E(x) will be to send a current I: forward into the termination at and a current Ib backward to the termination at Referring all current elements of I1 to the point I E@ -i-(I) 7 d f--rf Combining Equations 7 and 6, and summing the elements of Ifleads toy where lie-Wbb] When M and A2 are not equal, the condition necessarily found between the desired wave and the spurious mode waves .infie 1, .the forward current maynot the inazrtrrlurry is obtained.

The expression of Equation l1 is seen to be in the form of a Fourier transform, i. e., the forward current is ,equal to the constant k times the evaluation of the integral for the particular coupling distribution p(.\') (which evaluation is the Fourier transform of the coupling distribution). The quantity 0 is the periodic angle of :the Fourier ytransform, and when 0 is such .that the value of the transform is zero, the forward current will be zero.

For a specific example, consider the coupling distribution `of Fig. l which comprises eight equal Strength point couplings distributed over a total ,array length L. The Fourier transform with this distribution is well known to F=cos Q-I- cos -lcos -lcos 0 (.12)

The characteristic of Equation 12 is .plotted as curve 30 on Fig. 3, and it is seen to be a periodic function which passes through zero at specific values of 0, for lexample 7s1r, 1%1r, and 2%1r. These values, therefore, define vrthe specific relationships between the coupling length L and the phase velocity diiference'iunction for which the forward currentrofEquation 11 will be zero for the particular coupling distribution, in other words, the relationships for which maximum velocity discrimination is obtained against the wavelength M. Infinite velocity discrimination is obtained, therefore, when This velocity discrimination characteristic is shown as curve 31 on Fig. 3.

In every case the most critical spurious mode to be discriminated against is that mode which may be supported by the transducer multimode line which mode :has a lguide wavelength or a phase velocity nearest to, Vvbut not equal to, the guide wavelength or phase velocity of the .desired mode. Of the spurious modes noted above for the embodiment of Fig. 1, the guide wavelength in guide 10 of the TfEzl mode .is the nearest to :the guide wavelength of the TEcr mode. Thus, the coupling length L is chosen with k2 equal to the guide wavelength of the T1531 mode, to place the discrimination against this mode at the first point of innite decibel discrimination, i. e., at 0=7a1r. For a specific embodiment in accordance with Fig. 1, in which the mid-band operating frequency is 24,000 megacycles in a rectangular wave guide having an inside wide dimension 0.3.59 inch, the remaining parameters calcu- '58 lated ,accordance with `the equations :given -above would be: r=%6 inch TE10=0.676 inch and For eight .couplings lwith 0=%1r, the .coupling length L would be 6.26 inches or in the order of l0 times the TEol wavelength. All other spurious modes having less critical velocity differences will fall upon larger values of 0 and will likewise be discriminated against, as shown by curve 3l of Fig. 3, .so long -`as the mode possessing the greatest velocity yderence does not .cause 0, for the particular couplingvdistribution of Fig. l, to assume a value much greater .than .61r and .to fall within the region of the second major lobe centered around 711-.

.Special consideration has been reserved for the TMll mode. An linspection of the table vof Bessel functions given above will .indicate that the Tit/111 mode has the same guide wavemngth as the TEoi mode. These waves are known in the vart fas degenerate pairs. Other degenerate pa-irs in circular wave guides are seen to be the TMm and T1502, .the TMre and Tloa. Degenerate pairs are also found .in rectangular waveguides as will be seen from inspection of .Equation 2. ,Si-nce each wave of a degenerate pair has the same guide wavelength, something more than velocity Adiscrimination is required to discriminate between these Waves. In the embodiment of Fig. l, aper tures 16 will `excite all transverse electric modes in Aguide 1:0 but will excite no TM modes. Thus, the coupling mechanism itself discriminates against the TM mode in a circula-r wave guide. p

In this respect severa-l general comments can be made with regard A' to theV Vmode rdiscrimination effects of a rounded aperture coupling mechanism. When a rounded aperture fis placed in the sidewall of the rectangular guide, it will couple any TEmo mode in rectangular Vguide to any: TE wave in the circular guide, and conversely any TE refund to TEM in rectangular, but will discriminate against TM circular guide modes because the rectangular guide will not respond to longitudinal voltages impressed on the side wall. When .placed in the center of the wide side of a rectangular guide, a vrounded aperture will couple ,any .mode of round .guide (except TEOu) to TEmu in rectangular guide, and will couple any TM mode of round guide toany TEW in 4rectangular guide :if m is odd For TEon in arcircular guide, T'Ero will not be .excited but TEro will be excited in rectangular guide. This .type of coupling is therefore favorable for discriminating against TEoi power in round wave guide if the TMm is desired in the circular guide.

Thus, when degenerate pairs are involved, the coupling mechanism is chosen to couple to or from the desired mode .of rhepair and to discriminate against the undesired mode of the pair. For the particular pair involved in Fig. l, any coupling mechanism which is reponsive to at least the TEm Wave in rectangular guide, excites at least the TEM Wave in the circular guide, and discriminates against at least the TMm in kthc circular guide, will be satisfactory. Other cases illustrating the choice of the particular coupling in accordance with the invention will be demonstrated hereinafter with specific reference to Figs. 4, 6 and 8.

While the primary considerations of the invention are directed to energy transferred in the forward direction in line 10 to load lcircuit 20, it is of interest to consider the character of the energy .appearing at terminals 11 and 27. Clearly, all energy which is not transferred from guide 13 to guide 10 will 'be absorbed by termination 22. The exact amount ofthe energy which will be transferred into guide 1 0 is zyet to be considered in detail. Likewise, energy transferred into guide ,10 in the backward direction Iwill be absorbed by termination 21. However, since it is desirable to convert the maximum amount of energy in 9 line into the TEM mode for delivery to load 20, it is desirable that the backward current be made as small as possible. Referring again to the diagrammatic representation of Fig. 2, all elements of the backward current Ib may be referred to the point 21r L am Tte-2) dI 1,- 2 Z e da: (13) Combining Equation 13 with Equation 6 leads to L 2- I i21r I I,=kf L ,0(1) e (N M) dw (14) Making the substitution o' 1 1 -T(M+a) an equation identical to Equation 11 above is obtained. Thus, the velocity discrimination characteristic of Fig. 3 serves also as a representation of the directivity of the transducer of Fig. 1 except that 0 is a sum function of the relative velocities and It is thus seen that, insofar as conduction in the backward direction in guide 10 is concerned, the transducer of Fig. l possesses certain multimode directional coupling characteristics and that minor backward currents are inherently obtained when the coupling interval L is suiciently long.

The particular coupling distribution of Fig. l, comprising eight equal strength and equally spaced apertures, has been chosen to illustrate the principles of the invention because of its simplicity and because it serves to illustrate many of the necessary considerations. It should be noted, however, that any number of apertures, spaced equally, unequally, or spaced according to some predetermined function; of equal, unequal or tapered strength, may be used. In each and every case .the same considerations set forth above apply since each coupling distribution involves a Fourier transform from which the particular value of 0 is obtained. It should be noted that when n equal strength and equally spaced apertures are employed, the value of 0 for which the Fourier transform first passes through zero will always vbe Thus, as has beenrdemonstrated above for eight equally spaced and equal strength apertures, this value of 0 becomes %1r.

The number of coupling points, however, will determine the position of the second major lobe, shown on Fig. 3 as centered about 711-. As more coupling points are employed, the positionr of4 this lobe moves to larger values of 0. Consequently, when a very large velocity difference between any spurious mode to be discriminated against and the desired mode exists, it is desirable to Vemploy an increased number of coupling points.

The number and amplitude Ystrength of the couplings is of principal importance in determining the TEM power transferred from line 13 into TEU; 'power in-line 10. Thus, energy transferred'from line 13 to line 10 through the lrst left-hand aperture 16 experiences 9() degree' phase delay. This energy travels in the TE01 mode to the right along line 10 to the second coupling aperture 16 and part of this energy returns to line 13 with the further phase delay f` degrees. Thus, energy which goes from line 13 to line 10 and back to line 13 at a later coupling point arrives in line 13 out of phase with the TEio energy which travels straight through line 13. A summation of such components eventually results in cancellation of the TEio wave in line 13.

Designating the magnitude of the coupling through each of the apertures 16 as C, and assuming unit applied voltage at terminal 26, the voltage V1 in line 10 after the first coupling point may be expressed and the voltage E1 in line 13 as Upon passing the second coupling unit, these voltages become, respectively Vz=\/1-C2 V1+CE1=2CV 1-C2 (18) and E2=\/1-C2 E1-CVr=1-2C2 (19) Employing the transformation =sin w (20) Equations 18 and 19 may be expressed after n coupling units as En=cos nw (21) V1t=sin nw (22) Equations 21 and 22 may be rewritten in the form E=cos (n sin-1 C) (23) Vn=sin (n sin-1 C) (24) where m` is any odd integer.

It is desirable in certain applications, as pointed out above, to utilize a coupling array employing a plurality of apertures having different coupling factors. The desired power division may then'be determined as where E and V are thevoltages in line 13 and line 10, respectively, at the end of the series of couplings, and there are employed:

n1 apertures of individual coupling C1 n2 apertures of vindividual coupling C2 n3 apertures of individual coupling C3 nk apertures of individual coupling Ck and the transformation Ci-V-sin wi Cz=sin wz Ck=sin wk has' been employed in writing the above expression for '11 Eand V. Equations .25 and 26'n1ay be :rewritten in the form Again `complete jpower transfer of TEiu energy 4to TEur energy between 'the lines `will take place when the 'expression in the parenthesis of Equation 28 is equal to in which m may be any odd integer.

It should be noted that the `transducer of Fig. 1, like all of the transducers to be disclosed herein, is bilateral, i. e., the same power transfer will take place from line l@ -ito line 13 as has been described as taking place from line 13 to line 1). Assuming that load circuit 20 is therefore a transmission system delivering a plurality of modes including the TEor mode to terminal 12 of line 10, only that energy in the rDE01 .mode will be transferred into guide 13, appearing therein in the TEio mode,

while energy in the other modes in 'line 1.0 will be delivered to and absorbed by termination 21. Similarly, if source 25 is replaced by a system delivering ya large number of modes to terminal 26 of guide 13, and guide 13 is assumed to have such initial dimensions as to support these modes, only that power appearing in Yguide 13 in the TEm mode will be transferred to the TEo1 mode l in line 10.

In Fig. 4, a modification of .the transducer of Fig. 1 is shown in which coupling between the lines is provided by a divided aperture 35. vThe exact nature and the coupling characteristics to `be expected from a divided aperture such as 35 is fully disclosed and claimed in the copending application .of A. G. Fox, Serial No. 236,556, tiled July 13, 1951, now United States Patent No. 2,701,- 342, granted February l, 1955, and in-my copendng application, 'Serial No. 216,132, Vfiled March `17, 1951, now United States Patent No. 2,701,340, granted February l, 1955. it is suicient to state Ihere that aperture 35 is an elongated rectangular aperture of length L extending through the contiguous walls of .guides 32 a-nd 33, -as shown, and `is termed divided since extending parallel across the transverse or narrow dimension .of aperture 3S is a grid, comprising a plurality of dividers or -wlres 36. The dimensions and spacing Yof wires 35 are set forth in detail in the above-mentioned copending applications. pling between lines 32 and 33 which is effectively distributed to a substantial degree along the length L of the aperture. The Four-ier transform for such a .coupling distribution is known to be in which 0 is the periodic angle of the transform and The total velocity discrimination characteristic for all modes is shown as curve 37 on Fig. 5. It will be noted that this coupling distribution produces no subsequent major :lobes but rather I.the velocity discrimination chan This divided aperture provides a current cou- 12 acteristic continues to increase as the velocity difference between the `desired mode and the spurious modes vincreases.

'The same principles apply with corresponding advantages to each of the various geometric shapes Yof -divided apertures disclosed in my above-mentioned copencling application. These principles Yapply to each of the composite geometric shapes disclosed therein as well as to the basic geometric shapes of which the uniform rectangular coupling of Fig. 4 is an example.

When distributed Ycoupling is employed in the mode transducer, such .as the coupling provided by aperture 35 of Fig. 4, or when such a large number of discrete coupling points is employed that the coupling may be considered as substantially distributed, the criteria for complete power transfer between the desired modes 'is more conveniently expressed by the relations now to be defined.

Consider' again, therefore, Fig. 2 and particularly the TEoi current transmitted in the forward direction in line l and within a length interval so small that negligible power is transmitted between the lines l and 2. 'The envelope of the traveling wave in line 2 may be expressed wherein a represents the continuous coupling per unit length between the lines. Assuming a TEro wave of magnitude unity impressed on line 2, Equation 31 'becomes wherein L represents ,the distance over which :the cou- .pling is maintained.

Since the coupling length L of Fig. 4 is lfixed, being determined for maximum velocity discrimination against the lspurious modes, it is seen that the TEm wave magnitude in line 33, represented by Equation .313, declines cosinusoidally and the TEni wave magnitudes inline 32, represented by Equation 34, increases -sinusoidally I,as the distributed coupling strength a is increased. Complete power transfer from the TEM) inode in line 33 to the TEoi mode in line .3 2 takes :place when .the product 2aL=m1r radians, where m vis any odd integer, and repeats .cyclically as .a is further increased. .In other words, the "iT-E1n power .in line ,S3 and the TEM power in line 3,2 will be divided :in accordance with the ratio T @Zwwcos'l 2aL TE.,w site 2.1L (on) As fully disclosed in the above-mentioned copending applications, the current coupling factor a is directly dependent upon the transverse dimension of the divided aperture. Thus, this transverse dimension is chosen with respect 'to the already 'fixed longitudinal dimension L to provide any desired ratio of mode conversion. For complete conversion of all power, the transverse dimension is chosen to make oc equal to mbeing -any odd integer.

The vdivided aperture 3S of Fig. 4, like the rounded apertures .of Fig. 1, discriminates against the 'TMm mode of the .degenerate pair TEor and TM-ir. in general, the .mode discrimination characteristics of divided aperture 35 are identical v to those of an undivided rectangular aperture of the same shape and position. Thus, an uneach other.

divided rectangular aperture, long dimension parallel to the circular guide axis and placed in the narrow side of the rectangular guide, will couple TEM modes in rectangular guides to any TE wave of the round guide. Round guide TM waves will be discriminated against. When such a rectangular aperture is placed in the center of the wide side of the rectangular guide it will tend to couple TE round guide modes to the TEzo rectangular guide mode. The rectangular guide TEio modes are discriminated against by virtue of the hole placement in the center of the wide side of rectangular guide, while the round guide TM modes are discriminated against by virtue of the long narrow hole width which fails to intercept the longitudinal round guide currents.

' It should also be noted that the mode coupling characteristics of a coupling probe are similar to, but not identical with, those of the rectangular aperture. For example, a probe, radial in the circular guide and in the center of the wide side of the rectangular guide, will respond to any wave in the round guide except TED and will excite only dominant wave TEM) in the rectangular guide even if the TEzo mode could exist. Conversely, the probe will respond to the TEio mode in the rectangular guide and will excite all modes in the round guide, except TEQn, since here, as in'each case, the reciprocal relationship exists.

Fig. 6 illustrates a selective mode transducer, in accordance with the invention, in which the dominant mode TEio wave energy in rectangular guide 40 is transformed into TM11 wave energy in circular guide 41. The relative dimensions of guides 40, and 41 are identical to the relative dimensions of guides 13 and 10, respectively, of Fig. 1. The length of the coupling interval L is the same in both transducers. The principal diterence exists in the coupling means employed in Fig. 6 which discriminates against the TEoi mode of the degenerate pair TEoi and TM11. This coupling comprises a plurality of rectangular apertures 42, specifically eight as also employed in Fig. l for illustration, disposed with the long dimensions thereof perpendicular to the axis of guide 41 and in the wide wall of guide 40. Such an aperture will couple any mode of circular wave guide except the TEon modes to the TEm mode in the rectangular guide. It should be noted that a rectangular aperture, long dimension perpendicular to the round guide axis and placed in the small side of the rectangular guide, tends to have weak coupling for all modes because the longitudinal round guide currents do not suciently excite the rectangular guide.

Having thus described the basic principles and construction of the selective mode transducer with reference toseveral illustrative embodiments, several speciiic and useful applications of the transducers in` accordance with the invention will now be described. One such application, given here for purposesof illustration, is in the multichannel intelligence transmission systems. It has heretofore been demonstrated that a multimode transmission line may be used to transmit simultaneously a plurality of intelligence-bearing signal channels, each channel being transmitted by energy in different -modes or each in the same mode with dilerent guide excitation polarities.

In Fig. 7 is illustrated a particularly interesting example of the latter, wherein two signal channels are transmitted in a single round wave guide by cross-polarized TE11 Wave energy.4 Cross-polarized TE11 waves comprise, as shown inthe cross-sectional view of Fig. 7A, two electric elds 61 and 62 of TE11 energy polarized at right angles to Thesevwaves have been demonstrated to beK stablev andnot undesrably subject to cross-talk between theirrespective modulating signals. Thus, in Fig. 7, aktirst modulating signal from source 56 is impressed uponarv carrier signal'from source 57 in a lmodulator 53 and launched as TEio dominant mode energy'in guide 51.

A second modulating signal from source 55 is similarlyimpressed upon the carrier from source 57 by modulator 54 and launched as TE1o energy in guide 52. A portion of a narrow wall of guide S1 is coupled by a plurality of apertures 61 distributed over the interval L to the circular multi-mode guide 50 in a manner substantially identical to that already illustrated with reference to Fig. l. Guide 52 is similarly coupled by apertures 62 to guide 50 along a line displaced degrees around the circumference of guide 50 from the line of coupling of guide 51. The unused forward ends of guides 51 and 52 are terminated in a reectionless manner by terminations 60 and 59, respectively, each of electrical high loss material. The reverse end of guide 50 is similarly terminated in a reectionless manner by termination 58. 'Ihe relative cross-sectional dimensions of guides 51 and 50 and guides 52 and 50 are chosen in accordance with the invention so that the guide wavelength of the TE11 wave energy in guide 50 is equal to the TEio guide wavelength in both guides 51 and 52. In accordance with the considerations already noted with reference to Equations 1, 2 and 3, this requires the radius r of guide 50 to be times the wide dimension a of guides 51 and 52 in which the factor 1.84 is the Bessel function constant for the TE11 wave tabulated above.

All spurious or undesired modes which would ordinarily be capable of being supported in guide 50 are sub1 stantially suppressed by proportioning the length L of the coupling interval in accordance with Equation 4 above. Inasmuch as the TMoi mode would have a guide wavelength or a phase velocity most nearly equal to, but not the same as, the guide wavelength or phase velocity of the TE11 mode, the velocity diierence function of Equation 4is determined by the guide wavelengths of the desired TE11 mode and the TMm mode, as indicated by the relation shown on Fig. 7. Thus, the modulated- TEio energy in guide 52 will be transferred to a vertically polarized TE11 wave 102 in guide 50, as may be seen on the cross-sectional View Fig. 7A, and the modulated TEio energy in guide 51 will be transferred to a horizontally polarized TE11 wave 101 in guide 50.

At the receiving end of guide 50 a receiving station of structure identical to that of Fig. 7 will serve to separate and demodulate the two cross-polarized TE11 components. Assuming such a station, the location and orientation of the rectangular guides 51 and 52 around the cir`- cumference of the circular guide 50 serves to separate the two TE11 waves from each other; the velocity discrimination of the particular coupling intervals will serve to mitigate against coupling into unwanted modes. It should further be noted that the structure of Fig. 7 may serve for simultaneously transmitting and receiving. If, for example, modulator 53 is a demodulating apparatus, the vertically polarized TE11 energy 62 will serve for transmission of an intelligence signal to the station represented by Fig. '7, while the horizontally polarized TE11 energy 61 will serve for transmission away fro the station represented by Fig. 7.

In Fig. 8, a multimode channel system is illustrated in which the TE11 and TMoi modes are employed to transmit and/or receive separate intelligence-bearing signals.

The TE11 and TMm modes are of interest since they are the lowest order circular wave-guide modes. The overall system of Fig. 8 is substantially similar to the system shown in Fig. 7 and corresponding reference numerals have been employed to designate corresponding components. Guides 51 and 50, having the same relative cross-sectional dimensions a1 and r, respectively as corresponding guides of Fig. 7, are coupled, for the sake of variety of illustration, by a divided aperture 65 substan tially identical to the divided aperture 35 of Fig. 4, havingalengthL1. c f a 15 The T510 'output `of `modulator 54 is applied `to .guide 66 having the wide dimension .f1.2 thereof coupled to guide S along ,a longitudinal length L2 by rectangular apertures 67 `substantially identical to apertures ,52 of Fig. v6. Guide 66 is kterminated -in a reflectionless manner by termination 63. The dimension a, of guide 66 is proportioned with respect to the radius r of guide 50 so that the guide wavelength of the TEm wave energy in guide 66 will equal the guide wavelength of TMm wave energy in guide 50. Thus, in accordance with Equations 1, 2 and 3, the dimension .112 -is equal ,to

times `the radius r .of .guide 50 in which :the factor. .2.40 is the Bessel ifunction constant for Ythe TMm mode. As has already :been noted in connection with Fig. 6, rectangular apertures, such as apertures, 67, .having the :long dimension thereof :perpendicular to the axis of guide 50, may excite TM modes :including the TMm mode in .circular guide 50.

In determining the lengths L1 and L2 of each of the coupling distributions, the `velocity difference function of Equation 4 in each case are the same since aperture 65 must discriminate between the desired TEn mode and the `undesired TMm mode, -While lthe array `comprising apertures V6." -mustdiscriminate between the `desired TMm mode and the undesired TEn mode. However, L1 is not Yequal vto L2 since the value of -0 in each case is different. YIf divided aperture v65 is of the yshape illustrated 'in the drawing, l0 will be equal to 1r radians, `as demonstrated with reference to Fig. 4. On the other hand, the lvalue of 0 Afor the array comprising -apertures 67, assuming eight equal apertures, will -be equal to Z/s-nradia-ns, as demonstrated with reference to Fig. 1. The general vexpressions forthe lengths `L1 and L2 are indicated on Fig. I8. Thus, the selective mode transducer comprising guides v51 and 5t) `may serve to launch modulated TEM wave energy Vin guide 50 for a -transmitting operation or to select the 'PEu waveenergy from other mode energy in yguide 50 for a receivingoperation. Likewise, the mode transducer comprising guides 66 and 50 may serve to launch-or select TMoi wave energy forreither transmitting or receiving operations, respectively.

Itshould be noted that while in Fig. 8 guides "51 and 66 are illustrated as being-coupled to guide 50 along different 'longitudinal lines, this particular relationship is not necessary. lf `guide 51 is moved to couple along the same llongitudinal line `as guide 66, a TEn mode having 1a polarity at right `angles to the mode illustrated would be launched in guide t). In some respects, this modica- :tion is preferable since the spurious TE11 mode which `apertures 567 tend .to introduce must then :be of .diderent polarity from the .desired TEn modes introducedfby :aperture `65, and a .possible source of lcross-.talk is thereby eliminated.

ln Fig. 9 is shown .a multimode transducer by which each of 4a plurality of intelligence-bearing Vsignals maybe aunched in one of Vthe lower -order circular .electric wave modes. For example, on Fig. `9 three modulated 'l-Eio waves, designated, respectively Tdiol, TE1U2 and TEui, .are to be launched in a Ysingle circular wave guide '79 for 4long :distance transmission inthe "fl-3011, TfEozZ' and TEna3 modes, respectively. The TEtn portion of the transducer -ofFig 9 comprises a rectangular guide 7i! coupled kin the .manner heretofore described with reference to Figs. 1 .or :4 over an interval L1 to a circular guide 71 of radius 111. The forward end vof guide 70 and the backward end of guide .71 tare terminated in a retlectionless manner by vterminations 72 .and 73, respectively. 'The circular lguideV radius is Vincreased from the `radius r,of guide 71 to the radius r2 of guide 74 Lby a linear Vtaper .section Y'75. A .second :rectangular guide 76, terminated in a rellectionless manner Vat its ifor-ward .end by termination17:7, is coupled to guide 74 over an interval Lz, as heretofore described.

Guides 76 and 74 comprise the TEoz section of the transducer. Similarly, the radius of the transducer is again expanded by section 78 to the radius r3 of section 79 which comprises, together with rectangular guide .80, coupled thereto over `an interval L3, the TEoa section of the transducer. The forward end of guide 80 is terminated in a reflectionless manner by termination 81.

Each of the rectangular guides 70, 76 and 80 have equal wide dimensions a, chosen to support the dominant TEn) wave energy applied thereto. The radius r,L of guide 7l is chosen in accordance with considerations already fully detailed hereinbefore to render the guide wavelength of the TEoi mode therein equal to the wavelength of the TEm mode in guide 70. Similarly, the radius r2 of guide 74 is chosen yto render ,the guide wavelength of the TEoz energy `therein and the radius ra of guide 79 is chosen `to render the wavelength of the TEua energy therein, equal to the guide wavelength of the TEm wave energy in guides 76 or 80, respectively. Thus, in accordance with Equations 1, 2 and 3:

In guide 71 the mode having the nearest phase velocity to the desired TEor mode and, therefore, the critical mode in the determination of the velocity discriminating length L1, is the TEai mode. Similarly, in guides 74 Vand 79, the critical spurious modes are the TEzz and the TEza modes, respectively. The critical mode is in each case determined by an inspection of a portion above and below the listing of the desired mode on the Bessel function tables set out hereinbefore. Thus, the lengths L1, L2 and La, of the coupling distributions between guides 70 and 71, 76 and '74, and 80 and 79, are proportioned according yto Equation 4 to have the relations set out on Fig. 9 and to therefore discriminate between each desired mode and its critical spurious mode.

ln Fig. 10, an alternative mode multiplex system is shown in which a plurality of dominant mode TEio modulated signals of different frequencies are transformed to TEm circular electric wave Asignals at corresponding `frequencies. Thus, a rst modulated TEio wave at a lrst frequency, designated on Fig. v10 as TEoifl, is transformed into a first modulated TEoi wave at the same frequency, designated non Pig. -10 TEM/1. Similarly, dominant -mode waves at frequencies fz and is are converted into circular electric waves. The f1 portion tof the transducer comprises a rectangular guide having a wide dimension (1 terminated at its `for-ward end by :termination 8.6, and coupled over a longitudinal length Li to guide 87 of radius r1. Guide S7 is terminated in its backward .direction lby termination 96. The f2 portion of the ,transducer comprises a rectangular guide 88, having a wide dimension a2, :terminated at its fforward yend -by termination 89, and coupled over :a longitudinal length L2 -to guide 90 of radius The -a portion of fthe transducer lcomprises a rectangular guide 91, having a wide dimension as, terminated at its forward end by termination ,92, and .couipled over -a klongitudinal slength -Ls to guide 93 of radius r3. The transition ,between radius r, of Vguide 87 'to radius `rgfof Lguide 90 is accomplished .lay linear ltaper portion 94. vA similar lineaitaper portion Y is provided between `guides 9i) 4and 93. El" he dimensions 11,112 and q, for tec- -tangu-lar guides 85, 88 and :91 .are chosen to support kthe TiEm energy at `the frequencies fr, f2 and fa lin each, respectively. :lu the illustration of Fig. .10 f1 is assumed .to he va y frc-iguency higher than f3 Vand therefore -the .dimension-a1 of `guide 85 is substantially smaller than .thedimension aof guide 91.V For each frequency section r of the transducer, fthe .radius Vof the .circular guide member is in which the quantity 3.83 is the Bessel function constant for the TEnr wave. Again the length L for each frequency section of the transducer is chosen to discriminate between the desired TEoi wave in the circular guide of that section and the critical spurious mode TE31 therein. Therefore, the lengths of L1, L2 and L3 will be equal when measured in wavelengths but unequal in actual physical dimensions, since the relative guide wavelengths of the TEoi and TE31 waves will be different at the frequencies fr, f2 and fa.

While the principles of the invention have herein been demonstrated with reference to systems in which one guide is rectangular and the other is circular, inasmuch as present commercial interest appears to be directed to such systems, the principles of the invention are by no means limited to coupling between these particularly shaped wave guides. The related guides may be of the same geometrical cross-section. Any or all of these may well be elliptical, triangular, square, or of any other shape, as is Well known in the art.

For example, Fig. 1l illustrates a selective mode transducer, in accordance with the invention, in which the dominant mode TEm wave energy in a rectangular guide 97 is transformed into TE20 Wave energy in a second rectangular guide 98. The relative wide dimensions of guides 97 and 98 are chosen in accordance with Equation 2 above so that the cut-off wavelength for the TE1o mode in guide 97 equals the cut-olf wavelength for the TEzo mode in guide 98. This requires the wide dimension a, of guide 98 to equal twice the wide dimension a1 of guide 97. The narrow dimension of the guides may be equal.

The center of the narrow wall of guide 97 is coupled by a plurality of rectangular apertures 99, each disposed with the long dimension thereof parallel to the axis of the guide 97, to the center of the wide wall of guide 98. The apertures 99 are distributed along the length L chosen in accordance with Equation 4 above to discriminate between the desired TEzo mode in guide 98 and the undesired spurious TEio mode therein. As in the examples illustrated hereinbefore, the value of depends upon the exact number and strength of such apertures.

Apertures 99 will couple between the TEro mode in guide 97 and the TEzo mode in guide 98 and since the apertures 99 are located substantially exactly on the center line of the wide wall of guide 98, very little coupling will exist between the TE1o mode of guide 97 and the TEro mode of guide 98. Thus, the velocity discrimination as hereinbefore described is enhanced by the aperture coupling discrimination to make possible a very high degree of mode purity in the transfer of power from guide 97 to guide 98.

Several general comments can be made with regard to mode discrimination effects of aperture coupling between rectangular guides. If the guides of Fig. l1 were coupled by apertures in a common narrow wall, the TEio mode of guide 97 would couple to both TE10 and TEmo modes in guide 98. If the guides were coupled by apertures on the center line of a common wide wall, the TEio mode of guide 97 would couple only to the TEio mode of guide 98.

While the invention has been illustrated with reference toA shielded transmission lines, specifically of the wayeguide type, the principles of the invention apply to each type of electrical transmission system known in the art, including for example, the coaxial transmission line or the open two-wire transmission line. In each case the coupling between the related lines is obtained over the length specified herein by the particular coupling means typically employed for and in each of these transmission systems. The velocity of propagation of wave energy along any of these systems is regulated by the means known for each system for controlling the value of eective distributed inductance and capacitance of the lines in the same way that these parameters are controlled by the physical dimensions of the wave-guide transmission lines herein illustrated.

In all cases, it is understood that the above-described arrangements are simply illustrative of a small number of the many possible specific embodiments which can represent applications of the principles of the invention. Numerous and varied other arrangements can readily be devised in accordance with these principles by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

1. A selective mode transducer for multimode high frequency electrical transmission systems comprising a first shielded transmission line having a transverse crosssection, said line adapted to support wave energy in a plurality of modes of propagation including a rst mode, said line having a guide wavelength A, for said first mode, a second transmission line located adjacent said rst line and coupled with a given coupling distribution to said firstv line over a longitudinal length of said lines, said second line having a transverse cross-section of dimensions dilferent from those of said rst line cross-section, said dimensions of said second line providing a guide wavelength for a second mode of propagation therein substantially equal to said x1, said first mode and said second mode being different modes of wave energy propagation, said longitudinal length of coupling being substantially equal to wherein 0 is at least the value of the periodic angle of the Fourier transform of said coupling distribution at which the characteristic of thetransform first passes through zero, wherein )t2 is the guide wavelength of a mode of said plurality nearest said M, whereby wave energy propagated in said second mode in said second line will be transferred to said iirst mode in said first line and all other modes of said plurality will be suppressed in said first line.

2, A selective mode transducer for multimode high frequency electrical transmission systems comprising a main wave-guide transmission path for said energy, a plurality of auxiliary wave-guide transmission paths, each of said auxiliary paths being coupled with a given coupling distribution to a portion of said main path over a longitudinal length of said main path, each portion of saidmain path over which an auxiliary path is` connected havinga transverse cross-section and a given guide wave? length for a predetermined mode of wave energy propagation, each of said auxiliary paths having a'transverse cross-section of dimensions dierent from and adapted to support wave energy in a particular mode of propagation different from the transverse cross-section of and the predetermined mode in the portion to which that auxiliary path is coupled, each of said auxiliary paths having a guide wavelength for said particular mode of propagation therein substantially equal to said guide wavelength of said predetermined mode in the portion to which that auxiliary path is coupled, said longitudinal length of coupling for each portion being substantially equal to wherein 0 is at least the value of the periodic angle of the Fourier transform of said coupling distribution at which the characteristic of the transform first passes through zero, wherein M is said given guide wavelength for that portion, and wherein M is the guide wavelength of a mode of propagation in that portion of said main path nearest said M, whereby wave energy propagated in one of said particular modes in each of said auxiliary paths is transferred to one of said predetermined modes in said main path and all other modes in said main path are suppressed.

3. A selective mode transducer for multimode high frequency electrical transmission systems comprising a first transmission line, said line having a given guide Wavelength for a first mode of wave energy propagation, a second transmission line coupled to said first line over a longitudinal length of said lines, said second line adapted to support Wave energy in a plurality of modes including a second mode of propagation different from said first mode, said second line having a guide Wavelength for said second mode substantially equal to said guide Wavelength of said first mode in said first line.

4. A selective mode transducer for multimode high frequency electrical transmission systems comprising a rst transmission line, said line adapted to support a plurality of modes of wave energy propagation, said line having a guide wavelength M for predetermined one of said modes, a second transmission line located adjacent said first line and coupled with a given coupling distribution to said first line over a longitudinal length of said lines, said second line having a guide wavelength for a mode of propagation therein substantially equal to said' M, said longitudinal length of coupling being substantially equal to t wherein is at least the value of the periodic angle of the Fourier transform of said coupling distribution at which the characteristic of the transform first passes through zero, and wherein )t2 is the guide wavelength of a mode of said plurality nearest said M.

5. A selective mode transducer for multimode high frequency electrical transmission systems comprising a first shielded transmission line having a transverse crosssection, said line adapted to support a plurality of-modes of wave energy propagation, said line having a given guide wavelength for a predetermined one of said modes, a second shieldedy transmission line coupled to said iirst line over a longitudinal length of said lines, said second line having a transverse cross-section, the dimensions of said second line cross-section being different from the dimensions of said first line cross-section, said second line having a guide Wavelength for a mode of propagation therein substantially equal to said guide wavelength of said rst ine.

6. A selective mode transducer for multimode high frequency electrical transmission systems comprising a first shielded transmission line having a transverse crosssection, said line adapted to support a plurality of modes of Wave energy. propagation, said line having a guide wavelength M for a predetermined one of said modes, a second shielded transmission line located adjacent ksaid first line and coupled with a given coupling distribution to said rst line over a longitudinal length of said lines, said second line having a transverse cross-section of dimensions different from those of said first line cross-section, said second line having a guide wavelength for a mode ofv propagation therein substantially equal to said M, said longitudinal length of coupling being substantially equal to wherein 0 is at least the value of the periodic angle of the Fourier transform of said coupling distribution at which the characteristic of the transform first passes through zero, and wherein M is the guide wavelength of a modev of said plurality nearest said M.

7. A selective mode transducer for multimode high frequency electrical transmission systems comprising a first shielded'transmission line having a circular transverse cross-section, said line adapted to support a plurality of modes of wave energy propagation, said line having a given cut-off wavelength for a predetermined one of said modes, a second shielded transmission line located adjacent said first line and coupled to said iirst line over a longitudinal length of said lines, said second line having a rectangular transverse cross-section, said second line having a cut-off wavelength for a mode of propagation i therein substantialy equal to said cut-0E wavelength of said first line.

8. The combination according to claim 7 wherein the radial dimension of said first line is substantially i to a 7|' 7i' times one transverse dimension of said second line.

9. The combination according to claim 7 wherein the radial dimension of said first line is substantially to 7|' 7T times one transverse dimension of said second line.

l0. A selective mode transducer for multimode high frequency electrical transmission systems comprising a first transmission line, said line having a given cut-off wavelength for the dominant mode of Wave energy propagation therein, a second transmission line located adjacent saidiirst line and coupled to said first line over a longitudinal-length of said lines, said second line adapted to support wave-energy in the circular electric mode of propagation, said second line having a cut-off Wavelength for said circular electric mode substantially equal to said cut-olf wavelength of said dominant mode in said rst line.

1l. A selective mode transducer for multimode high frequency electrical transmission systems comprising a rst shielded transmission line having a circular transverse cross-section, said line adapted to support at least the TEoi and TM 1i modes of wave energy propagation, said line having a given guide wavelength for said modes, a second shielded transmission line located adjacent said first line and coupled to said first line over a longitudinal length of said lines, said second line having a rectangular transverse cross-section, the wide dimension of said second line Cross-section being substantially equal E to s times the radial dimension of said iirst line cross-section whereby the guide Wavelength for the TEio mode of propagation in said second line substantially equals said given wavelength of said first line.

12. The combination according to claim 1,1 wherein saidlines are coupled by means responsive to one of said modes in said first line and excluding the other of said modes therein.

13V. A selective mode transducer for multimode high frequency electrical transmission systems comprising a first shielded transmission line having a circular transverse cross-section, said line adapted to support at least the TEoi and TM11 modes of Wave energy propagation, said line having a guide wavelength M for saidl modes, a second shielded transmission line of rectangular transverse crosssection` located adjacent said first line and coupled with a given coupling distribution to said first line over a longitudinal length of said lines, said second line having a guide wavelength for the TE1o therein substantially equal to said M, said longitudinal length of coupling being substantially equal to wherein is at least the value of the periodic angle of the Fourier transform of said coupling distribution at which the characteristic of the transform first passes through zero, and wherein A2` is the guide wavelength of the TEsi in said rst line.

14. A selective mode transducer for multimode high frequency electrical transmission systems comprising a lirst shielded transmission line, said line having a given cut-ol wavelength for a rst mode of wave energy propagation, a second shielded transmission line having a portion of its length contiguous a portion of said iirst line, means coupling said lines over a longitudinal length thereof comprising a common shield in said contiguous portion having aperture means therethrough, said second line adapted to support wave energy in a plurality of modes including a second mode of propagation different from said first mode, said second line having a cut-oil wavelength for said second mode substantially equal to said cut-off wavelength of saidfirst mode in said lirst line.

15. The combination according to claim 14 wherein said aperture means comprises a plurality of apertures distributed along a line parallel to the axis of said shielded lines.

16. The combination according to claim 14 wherein said aperture means comprises a plurality of apertures distributed along a line parallel to the axis of said shielded lines, and wherein each of said plurality of apertures is rectangular in shape and is positioned in said shield with the longer dimension of said rectangle parallel to the axis of said shielded line.

17. The combination according to claim 14 wherein said aperture means comprises a plurality of apertures distributed along a line parallel to the axis of said shielded lines, and wherein each of said plurality of apertures is rectangular in shape and is positioned in said shield with the longer dimension of said rectangle perpendicular to the axis of said shield.

18. The combination according to claim 14 wherein one of said lines is rectangular in cross-section and wherein said common shield portion includes the narrower wall of said rectangular line.

19. The combination according to claim 14 wherein one of said lines is rectangular in cross-section and wherein said common shield portion includes the wider wall of said rectangular line.

20. A selective mode transducer for multimode high frequency electrical transmission/systems comprising a first shielded transmission line, said line adapted to support a plurality of modes of wave energy propagation, said line having a guide wavelength M for a predetermined one of said modes, a second shielded transmission line having a portion of its length contiguous a portion of said first line, means coupling said second line with a given coupling distribution to said lirst line over a longitudinal length L of said lines, said coupling means comprising a common shield in said contiguous portion having aperture means therethrough, said second line having a guide wavelength for a mode of propagation therein substantially equal to said M, said longitudinal length of coupling being substantially equal to wherein 0 is at least the value of the periodic angle of the Fourier transform of said coupling distribution at which the characteristic of the transform lirst passes through zero, and wherein k2 is the guide wavelength of a mode of said plurality nearest said M.

2l. The combination according to claim 20 wherein said aperture means comprises n apertures distributed along a line parallel to the axis of said shield and wherein 0 is substantially equal to 22. The combination according to claim 2O wherein said aperture means is a divided rectangular aperture and wherein 0 is substantially equal to 1r.

23. The combination according to claim 20 wherein said aperture means provides a substantially distributed coupling per unit length along said length L equal substantially to EL@ 2L wherein m is any odd integer.

24. The combination according to claim 20 wherein said aperture means comprises m11 2 sin1 C' coupling apertures each having substantially a coupling factor C, wherein m is any odd integer.

25. A selective mode transducer for multimode high frequency electrical transmission systems comprising a first shielded transmission line having a rectangular transverse cross-section, said line adapted to support a plurality of modes of wave energy propagation, said line having a given cut-oi wavelength for a predetermined one of said modes, a second shielded transmission line located adjacent said first line and coupled to said lirst line over a longitudinal length of said lines, said second line having a rectangular transverse cross-section, the wider dimension of said irst line cross-section being twice the wider dimension of said second line cross-section whereby said second line has a cut-olf wavelength for a mode of propagation therein substantially equal to said cut-off wavelength of said rst line.

26. A selective mode transducer for multimode high frequency electrical transmission systems comprising a main transmission path for said energy, a plurality of auxiliary transmission paths, each of said auxiliary paths being coupled to a portion of said main path over a longitudinal length of said main path, each portion of said main path over which an auxiliary path is coupled having a given cut-01T wavelength for a predetermined mode of wave energy propagation, each of said auxiliary paths adapted to support wave energy in a particular mode of propagation diterent from the predetermined mode in the portion to which that auxiliary path is coupled, each of said auxiliary paths having a cut-o wavelength for said particular mode of propagation therein substantially equal to said cut-off wavelength of said predetermined mode in the portion to which that auxiliary path is coupled.

27. A selective mode transducer for multimode high frequency electrical transmission systems comprising a main shielded transmission line of circular cross-section, said line adapted to support a plurality of modes of wave energy propagation, a plurality of auxiliary shielded transmission lines of rectangular cross-section, each of said lines being coupled to a portion of said main line over a longitudinal length of said main line, said main line having given cut-ot wavelengths for predetermined ones of said modes, each of said auxiliary paths adapted to support wave energy in a particular mode of propagation different from the predetermined modes in said main line, each of said auxiliary paths having a cut-olf wavelength for said particular mode of propagation therein substantially equal to said cut-off of a predetermined mode in said main line.

28. The combination in accordance with claim 27 wherein said auxiliary lines are each coupled to said main line along lines displaced around the circumference of 23 saidsmainfline whereby the same mode of wave energy is `launched inV said main line by each auxiliary line but with different relative polarities therein.

29. The combination in accordance with claim 28 wherein the radial dimension of said main line is substantially ]l to 2. 7|' Il' times one transverse dimension of an auxiliary line.

30. The combination in accordance with claim 27 wherein a part of said plurality is coupled to said main line by means responsive to one group of modes of wave energy propagation while excluding another group of modes, and wherein the remainder of said plurality are coupled by means responsive to a mode included in said other group and excluding a mode included in said one group.

31. The combination in accordance with claim 30 wherein the radial dimension of said main line is substantially l to g 7l' 7| times one transverse dimension of said part andsubstantially E to Il 7l' times one transverse dimension of said remainder.

32. A selective mode transducer for multimode high frequency electrical transmission systems comprising a main transmission path for said energy, Said main path comprising a connected plurality of circular crosssectional shielded transmission line portions, a plurality of auxiliary shielded transmission lines of rectangular cross-section, each of said auxiliary lines being coupled to a corresponding one of said main path portions over a longitudinal length of said portion, each portion of said main path having a given cut-off wavelength for a predetermined mode of wave energy propagation, each of said auxiliary lines adapted, to support wave energy in a particular mode of propagation` different from the prede- 24 termined mode in its corresponding portion, each of said auxiliary paths having a cut-off wavelength for said particular mode of propagation therein substantially equal to said cut-off of said predetermined mode in its corresponding portion.

33. The combination in accordance with claim 32 wherein the wide dimensions of all of said auxiliary lines are substantially equal, wherein the radial dimension of one portion of said main path is substantially times said transverse dimension, and wherein the radial dimensionsA of subsequent portions of said main path are integral multiples larger than a value in said range of .3. to 7l' 'Il' times said transverse dimension.

34. The combination in accordance with claim 32 wherein each of said auxiliary lines is adapted to support the dominant mode of wave energy at successively different frequencies and wherein the radial dimension of each portion of said main path is to .4. 'Il' 7l' times one transverse dimension of its corresponding auxiliary line.

References Cited in the file of this patent UNITED STATES PATENTS 2,512,191 Wolf June 20, 1950 2,514,779 Martin June 11, 1950 2,562,281 Mumford July 31, 1951 2,566,386 Varian Sept. 4, 1951 2,615,982 Zaslavsky Oct. 28, 1952 2,643,298 Arnold June 23, 1953 FOREIGN FATENTS 976,704' France Nov. 1, 1950 

